Exhaust gas from vehicles powered by gasoline engines is typically treated with one or more three-way conversion (TWC) automotive catalysts, which are effective to abate nitrogen oxides (NOx), carbon monoxide (CO), and hydrocarbon (HC) pollutants in the exhaust gas of engines operated at or near stoichiometric air/fuel conditions. The precise proportion of air to fuel which results in stoichiometric conditions varies with the relative proportions of carbon and hydrogen in the fuel. An air-to-fuel (A/F) ratio is stoichiometric when complete combustion of a hydrocarbon fuel, such as gasoline, to carbon dioxide (CO2) and water occurs. The symbol λ is used to represent the result of dividing a particular A/F ratio by the stoichiometric A/F ratio for a given fuel, so that: λ=1 is a stoichiometric mixture, λ>1 is a fuel-lean mixture, and λ<1 is a fuel-rich mixture.
Gasoline engines having electronic fuel injection systems provide a constantly varying air-fuel mixture that quickly and continually cycles between lean and rich exhaust. Recently, to improve fuel-economy, gasoline-fueled engines are being designed to operate under lean conditions. Lean conditions refers to maintaining the ratio of air to fuel in the combustion mixtures supplied to such engines above the stoichiometric ratio so that the resulting exhaust gases are “lean,” i.e., the exhaust gases are relatively high in oxygen content. Lean burn gasoline direct injection (GDI) engines offer fuel efficiency benefits that can contribute to a reduction in greenhouse gas emissions, carrying out fuel combustion in excess air. A major byproduct of lean combustion is NOx, the after-treatment of which remains a major challenge.
Emission of nitrogen oxides (NOx) must be reduced to meet emission regulation standards. TWC catalysts typically comprise a platinum group metal supported on an oxygen storage component and/or a refractory metal oxide support, and, optionally, an additional platinum group metal component supported on a second refractory metal oxide support or a second oxygen storage component. TWC catalysts, however, are not effective for reducing NOx emissions when the gasoline engine runs lean because of excessive oxygen in the exhaust gas. Two of the most promising technologies for reducing NOx under an oxygen-rich environment are urea selective catalytic reduction (SCR) and the lean NOx trap (LNT). Urea SCR systems require a secondary fluid tank with an injection system, resulting in added system complexity. Other concerns for urea SCR include urea infrastructure, the potential freezing of urea solution, and the need for drivers to periodically fill the urea solution reservoir.
Gasoline engines, particularly lean-burn gasoline engines, offer significant potential for improving fuel efficiency and reducing CO2 emissions. Three-way conversion (TWC) catalysts operating under lean conditions can generally perform HC oxidation, but the lightoff temperature is generally above 300° C. The engine-out temperature during lean excursion can be much lower than during stoichiometric operation, which poses a challenge in hydrocarbon (HC) conversion. TWC catalysts do not efficiently convert hydrocarbons at low temperatures (e.g. below 250° C.). Further, in lean-burn gasoline engines, NOx reduction is a challenge, because TWC catalysts cannot convert NOx under lean conditions. One of the exhaust architectures for lean-gasoline applications is the passive NH3—SCR system, which involves the use of an upstream catalyst to generate ammonia (NH3) (during fuel-rich conditions) for use by a downstream NH3—SCR for NOx reduction. Generation of NH3 over the upstream catalyst is the most important aspect of the passive NH3 approach, and increasing the conversion efficiency of engine-out NOx to NH3 is the key factor for improved NOx reduction efficiency. Maximizing engine-out NOx to NH3 conversion is also critical for improved fuel efficiency because NH3 generation consumes fuel.
To meet current governmental emissions regulations, there is a need for a technology that addresses both hydrocarbon (HC) conversion under lean conditions at low temperature and NOx emissions and does not negatively impact NH3 formation in gasoline engine applications.